Abstract: Obstructive sleep apnea (OSA) is a serious medical condition that affects an estimated 15 million US adults and is characterized by repeated episodes of upper airway obstruction, recurrent arousals and episodic oxyhemoglobin desaturations during sleep. These abnormalities contribute to subclinical alterations in vascular function that increase the risk of cardiovascular disease morbidity and mortality. In contrast, exercise decreases the risk of cardiovascular events and the promotion of physical activity continues to be at the top of our national public health agenda, as seen in the publication of the 1996 report of the US Surgeon General on physical activity and health. While the exact mechanisms for this protective benefit are not entirely clear, there is good evidence that exercise confers cardioprotection through its direct impact on vascular endothelial function. The American College of Sleep Medicine recommends exercise as a behavioral treatment option for OSA. Yet, most clinical trials upon which this recommendation is based have focused on establishing the effectiveness of lifestyle change (e.g., dietary induced weight loss and increased physical activity participation) for improving OSA severity in obese individuals, whereas less attention has been given to whether OSA moderates the effects of exercise on the cardiovascular disease substrate. Our overarching hypothesis is that OSA attenuates the beneficial effect of exercise on vascular function in obese individuals suffering from untreated OSA. The aims of the study are to 1) examine parameters of vascular function in obese persons with and without OSA, 2) evaluate the effects of an acute bout of exercise on brachial artery flow mediated dilation in obese persons with and without OSA, and 3) examine the effects of 6 weeks of exercise training on vascular function among those with and without OSA. Our data will be immediately useful in clinical practice and inform how we prescribe exercise and implement lifestyle changes to reduce cardiovascular disease risk in OSA patients.